O -phenylene octamers as surface modifiers for homeotropic columnar ordering of discotic liquid crystals

Article abstract of DOI:10.1021/ja4087853

Large-area homeotropic columnar ordering of π-conjugated discotic liquid crystals (LCs) is crucial for certain device applications but generally hard to achieve. Here we report polymeric o-phenylene octamer poly-1 and its monomer 1 as the first surface modifiers for homeotropic columnar order of a variety of discotic LCs up to a macroscopic length scale. Their octameric o-phenylene parts are known to fold helically into a cylinder that is reminiscent of a π-stacked column of discotic LCs. Through-view X-ray diffraction patterns of 1 suggested that this molecule adheres to the glass substrate and directs its cylindrical axis perpendicular to the glass surface. This "face-on" orientation likely nucleates the homeotropic columnar order of discotic LC materials.

Full text of DOI:10.1021/ja4087853

Communication
pubs.acs.org/JACS
o‑Phenylene Octamers as Surface Modifiers for Homeotropic
Columnar Ordering of Discotic Liquid Crystals
Takashi Kajitani,*,†,‡ Yuki Suna,† Atsuko Kosaka,†,‡ Terutsune Osawa,†,# Shigenori Fujikawa,†
Masaki Takata,§ Takanori Fukushima,*,†,‡ and Takuzo Aida*,†,#,∥
†RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
‡Chemical Resources Laboratory, Tokyo Institute of Technology, R1-1 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
#Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-8656, Japan
§RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan
∥RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
S
* Supporting Information
might be miscible with TP-based LC materials. Thus, o-
ABSTRACT: Large-area homeotropic columnar ordering
of π-conjugated discotic liquid crystals (LCs) is crucial for
certain device applications but generally hard to achieve.
Here we report polymeric o-phenylene octamer poly-1 and
its monomer 1 as the first surface modifiers for
homeotropic columnar order of a variety of discotic LCs
up to a macroscopic length scale. Their octameric o-
phenylene parts are known to fold helically into a cylinder
that is reminiscent of a π-stacked column of discotic LCs.
Through-view X-ray diffraction patterns of 1 suggested
that this molecule adheres to the glass substrate and directs
its cylindrical axis perpendicular to the glass surface. This
“face-on” orientation likely nucleates the homeotropic
columnar order of discotic LC materials.
phenylene octamer 1 carrying hexadecyl chains via an ester
linker at both ends (Figure 1a) was designed and mixed with TP8
(Figure 1e). We found that, up to a mixing ratio of 30 wt%,
compound 1 neither destabilizes the LC mesophase of TP8 nor
changes its packing geometry.7 These observations indicate that
1 is immiscible with TP8 at a molecular level and phase-separated
macroscopically, contrary to our expectation. However, we
noticed that, in the presence of this immiscible additive, the LC
columns of TP8 entirely align homeotropically. With this
serendipitous finding, we supposed that 1 might efficiently
nucleate such a particular columnar orientation at a glass/LC
interface. In order to ensure this interesting possibility, we
synthesized poly-1 (Figure 1b),8 which likely possesses a larger
adhesivity than monomer 1 toward the glass surface. In fact, as
highlighted in the present Communication, poly-1 modifies the
glass surface more efficiently for homeotropic columnar ordering
of TP8. Furthermore, this function also operates for other
discotic LC materials.
ith the increasing importance of soft electronic devices,
Wparticular attention has been paid to π-conjugated
discotic liquid-crystalline materials with a large-area unidirec-
tional structural order.1 However, in contrast with the case of
rod-like liquid crystal (LC) molecules that can uniformly align
horizontally on a rubbed polyimide film,2 an essential issue to
consider is the lack of general strategies to realize orientational
control of discotic LC molecules.3 Especially, homeotropic
columnar ordering of discotic LCs, which have many potential
applications, remains a difficult task.4 Here we report that
polymeric o-phenylene octamer poly-1 (Figure 1b) and its
monomer 1 (Figure 1a) serve as the first surface modifiers that
can nucleate large-area columnar ordering of discotic LC
materials without deteriorating their original phase features.
How to manipulate interfacial phenomena remains a big
challenge in chemistry and materials science. The present
finding sheds light on this fundamental issue.
Recently, we reported that terminally functionalized oligo-
meric o-phenylenes are a new class of stimuli-responsive
foldamers5 that adopt a cylindrical architecture with a 31-helical
conformation.6 Because of this molecular structure, these
cylindrical motifs, when projected along their longer skeletal
axis (Figure 1d), look like triphenylene (TP, Figure 1f). Hence,
we envisioned that this cylindrical motif, if properly designed,
In differential scanning calorimetry (DSC; Figure S1a),9 poly-
1 upon heating showed two glass transition events at 72 and 151
°C. As expected, the two glassy states displayed a featureless X-
ray diffraction (XRD) profile (Figure S1b,c).9 Thus, poly-1 itself
is substantially an amorphous material. Next, poly-1 was mixed
with TP8. In DSC, TP8 containing 10 wt% of poly-1, for example,
displayed a mesophase at 65−85 °C upon heating and 82−42 °C
upon cooling (Figure S4a).9 These phase behaviors are virtually
identical to those of TP8 alone (Figure S3a and Table S1).9,10
When this poly-1-containing LC material in a glass capillary was
exposed to a synchrotron X-ray beam, the through-view XRD
pattern showed characteristic diffractions with d-spacings of 2.03,
1.17, and 1.01 nm (Figure 2a), whose ratio is consistent with that
required for hexagonal packing (1:√3:2). The observed d-
spacings are substantially the same as those of TP8 alone (Figure
S3b).9 These observations indicate that poly-1 is phase-separated
from the hexagonal columnar (Colh) LC mesophase of TP8.11
Although phase-separated poly-1 is not detectable due to its
amorphous nature, phase separation was also supported by the
Received: August 24, 2013
Published: September 18, 2013
© 2013 American Chemical Society
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Figure 3. (a) OM and POM (inset) and (b) conoscopic image of a LC
film of TP8 containing 10 wt% of poly-1. (c) POM of a LC film of TP8
containing 3 wt% of poly-1. (d) POM of a LC film of TP8 alone. (e) OM
and POM (inset) of a LC film of TP8 containing 10 wt% of 1. (f) OM
and POM (inset) of TP8 on a glass substrate spin-coated with poly-1. All
LC samples were heated once to their isotropic melting point, allowed to
cool to 70 °C, and then subjected to microscopy. Scale bars for (a) and
(c)−(e) represent 100 μm, and that for (f) denotes 50 μm.
Figure 1. Molecular structures of (a) monomeric (1) and (b) polymeric
(poly-1) o-phenylene octamers and (c) o-phenylene tetramer (2).
Molecular structures of discotic LC molecules derived from (e)
triphenylene (TP8 and TP12), (g) phthalocyanine (Pc), and (h)
hexabenzocoronene (HBC). CPK models of (d) a hydrogen-terminated
octamer of 1,2-dimethoxybenzene and (f) hexamethoxytriphenylene.
texture, typical of Colh LCs, develops entirely. In contrast, its
polarized optical micrograph (POM) image was completely dark,
without any birefringence (Figure 3a).4j,13 From these together
with its conoscopic interference image (Figure 3b), we conclude
that, upon mixing with poly-1, the hexagonally packed LC
columns of TP8 align homeotropically with respect to the glass
substrate. Even when the doping level of poly-1 was reduced to 3
wt%, the POM image remained dark (Figure 3c). However, when
the content of poly-1 was below this level, the material, just as
TP8 alone (Figure 3d), was partially birefringent in POM,
indicating the occurrence of incomplete homeotropic columnar
ordering (Figure S5).9
Of interest, not only TP8 and its longer side-chain version
TP12 (Figures 1e and S10)9 but also other discotic LC molecules
having, e.g., phthalocyanine (Pc, Figures 1g and S11)9 and
hexabenzocoronene (HBC, Figures 1h and S12)9 cores adopt a
homeotropic columnar order in the presence of poly-1. The
result for HBC is noteworthy. As evidenced by a heavily
birefringent POM texture in Figure S12e,9 HBC alone in the LC
state has a strong preference for horizontal columnar ordering
with respect to the glass substrate. However, such a birefringent
texture disappeared almost completely when HBC was mixed
with 10 wt% of poly-1 (Figure S12f).9 Just like the case with
monomer 1, none of the DSC and XRD profiles of these discotic
LCs were deteriorated by poly-1 (Figures S10−S12 and Table
S1),9 indicating their immiscibility with poly-1.
Figure 2. XRD patterns of bulk samples of (a) TP8 containing 10 wt% of
poly-1 and (b) TP8 containing 10 wt% of 1 at 70 °C on cooling from
their isotropic melts. The samples were placed in a glass capillary (φ =
1.5 mm). Values in parentheses indicate Miller indices. Insets show
magnified XRD patterns in a wide-angle region. Based on the hkl = 100
diffraction peaks, lattice parameters were determined as 2.34 and 2.35
nm for poly-1-containing TP8 (10 wt%) and 1-containing TP8 (10 wt
%), respectively.
fact that the enthalpy changes associated with the crystal-to-Colh
and Colh-to-isotropic melt phase transitions, normalized based
on the contents of TP8, remain intact upon the mixing of poly-1
(Table S2).9 Figure 3a shows an optical micrograph (OM) of a 2-
μm-thick film12 of this mixture at 70 °C, where a dendritic
One may wonder why poly-1 induces homeotropic columnar
orientation of these LC materials. We found that the
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homeotropic orientation becomes more difficult for thicker film
samples. For example, when the content of poly-1 in TP8 was
fixed at 10 wt%, 25- and 50-μm-thick LC films, sandwiched
between glass plates, entirely displayed a dark-field POM image
(Figure S8a,b).9 However, a 100-μm-thick LC film was
birefringent (Figure S8c).9 This tendency indicates a possibility
that a nucleation event, leading to homeotropic columnar order,
is initiated by the adsorption of poly-1 onto the glass/LC
interface. Note that a 50-μm-thick LC film of TP8 became
birefringent when the content of poly-1 was reduced to 3 wt%
(Figure S6).9 Therefore, an adsorption equilibrium likely exists
and determines the amount of adsorbed poly-1 eligible for the
nucleation event. As briefly described in the introductory part,
monomer 1 is inferior to poly-1 in its ability to induce
homeotropic columnar order (Figures 2b and 3e). In fact,
when the content of 1 in TP8 was 10 wt%, even a 25-μm-thick LC
film was partially birefringent (Figure S9a).9 This trend seems
reasonable, since monomer 1 in such an adsorption equilibrium
is less advantageous than poly-1, having multiple polar ester and
urethane units capable of interacting with silanol groups on the
glass surface. Accordingly, when a glass substrate pretreated with
1,1,1,3,3,3-hexamethyldisilazane (HMDS) to protect the silanol
groups was employed, even a thin LC film (2 μm) of TP8, though
containing 10 wt% of poly-1, displayed a birefringent POM
texture (Figure S7).9
Although monomer 1, as a surface modifier, is inferior to poly-
1, this compound gave an important notion as to why these
cylindrical motifs drive the homeotropic columnar order of
discotic LC materials. As observed by DSC (Figure S2a),9
monomer 1, upon heating, showed a phase behavior featuring a
glassy state (<57 °C), a mesophase (57−91 °C), a soft-crystalline
phase (91−109 °C) displaying multiple XRD peaks (Figure
S2b),9 and an isotropic melt (>109 °C). In POM, the mesophase
displayed a birefringent texture (Figure S2c).9 Powder XRD
analysis of the mesophase at, e.g., 80 °C in a glass capillary
displayed four broad diffraction peaks with d-spacings of 3.08,
1.08, 0.52, and 0.40 nm (Figure 4a). Judging from the X-ray
crystal structure of an o-phenylene octamer analogous to 1,6a the
XRD peaks with d-spacings of 1.08 and 0.40 nm are attributable
to the molecular length and helical pitch, respectively (Figure
4b). Provided that the hexadecyl chains of 1 are folded along the
cylindrical molecular axis, the d-spacing of 3.08 nm agrees well
with a calculated molecular diameter (Figure S2d).9 Further-
more, the diffraction peak with a d-spacing of 0.52 nm most likely
arises from the side chains in the molten state. We noticed that
the intensity ratio (R) of diffractions, Id=3.08nm/Id=1.08nm, which
was 3.1 for the bulk state of 1, increased remarkably to 4.7 when
the sample was processed into a thin film (25 μm) sandwiched
between glass plates and exposed to a synchrotron X-ray beam
from its perpendicular direction (Figure 4c,d). However, this
change became less explicit when the film was made thicker,
where R eventually dropped to 3.1 when the film thickness
exceeded 50 μm (Figure 4e). Therefore, 1 adsorbed on the glass
surface aligns anisotropically. Considering the dimensional
aspect, adsorbed 1 most likely adopts a “face-on” geometry by
directing its cylindrical axis perpendicular to the glass surface
(Figure 4c). We consider that, in the LC media of TP8, TP12, Pc,
and HBC, this particular orientation possibly occurs at a glass/
LC interface, probably by an interaction of the polar ester termini
of 1 (and poly-1) with the silanol groups, and nucleates the
homeotropic columnar order.4d In this nucleation event, the
cylindrical shape of 1 plays a crucial role, because no
homeotropic columnar order of TP8 was driven by o-phenylene
Figure 4. (a) XRD pattern of a bulk sample of 1 at 80 °C on heating in a
glass capillary (φ = 1.5 mm). Schematic illustrations of (b) molecular
structure of 1 and (c) experimental setup for obtaining through-view
XRD images of 1. (d) XRD patterns of 1 measured at 80 °C on cooling
from its isotropic melting poing when sandwiched between two glass
plates using 25-, 50-, and 100-μm spacers and in a glass capillary (φ = 1.5
mm). (e) Plots of film thickness versus intensity ratio (R) of diffraction
peaks, Id=3.08nm and Id=1.08nm, observed at 80 °C.
tetramer 2 (Figures 1c and S13),9 which is incapable of folding
into a stable cylinder (Figure S13f).6a
Finally, we would like to emphasize that poly-1 functions
properly on a glass surface simply by spin-coating. For example, a
glass substrate was spin-coated beforehand with poly-1 using its
CHCl3 solution and then heated at 100 °C for annealing. Next,
on this spin-coated glass surface, TP8 was heated once to its
isotropic melting point and then cooled to 70 °C. As shown in
Figure 3f, even an open LC film14 thus processed entirely showed
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displayed a dendritic texture in OM.
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In summary, similar to the case with a rubbed polyimide film
for horizontal ordering of rod-shaped LC molecules,2,15 poly-1, a
polymeric o-phenylene octamer, is the first glass-surface modifier
that nucleates large-area homeotropic columnar ordering of
discotic LC materials. Homeotropically ordered columnar LCs
have the potential for many applications, such as thin-film
organic solar cells1d−g and ultra-high-density memory devices.4i
However, reported examples rely on a particular design of the
mesogenic cores and/or the side chains of discotic LCs. In
contrast, the present surface-modification approach using poly-1
could be applicable to a wide variety of discotic LC molecules and
thus expands the range of their applications.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Details of synthesis and characterization of 1, 2, and poly-1, and
DSC, POM, and XRD data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
kajitani@res.titech.ac.jp
fukushima@res.titech.ac.jp
aida@macro.t.u-tokyo.ac.jp
(6) (a) Ohta, E.; Satoh, H.; Ando, S.; Kosaka, A.; Fukushima, T.;
Hashizume, D.; Yamasaki, M.; Hasegawa, K.; Muraoka, A.; Ushiyama,
H.; Yamashita, K.; Aida, T. Nat. Chem. 2011, 3, 68−73. (b) Ando, S.;
Ohta, E.; Kosaka, A.; Hashizume, D.; Koshino, H.; Fukushima, T.; Aida,
T. J. Am. Chem. Soc. 2012, 134, 11084−11087.
(7) When TP8 was doped with monomer 1 at >30 wt%, DSC showed
complicated phase behaviors (Figure S4d).9 For example, at the doping
level of 40 wt%, the phase diagram obtained upon heating displayed an
unknown phase below 54 °C, a crystalline phase at 54−65 °C, and a two-
phase region at 82−103 °C involving the isotropic phase of TP8 and
mesophase of 1.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by KAKENHI (24350055 and
23750170). The synchrotron X-ray diffraction experiments
were performed at the BL45XU in the SPring-8 with the
approval of the RIKEN SPring-8 Center (proposal 20090053 and
20100062).
(8) By means of size exclusion chromatography using polystyrene
standards, number-averaged molecular weight (Mn) and polydispersity
index (Mw/Mn) of poly-1 were roughly estimated as 5.3 × 104 g/mol
(degree of polymerization, DP = 28) and 1.6, respectively.
(9) See Supporting Information.
(10) When TP8 was doped with poly-1 at >20 wt%, DSC showed
complicated phase behaviors (Figure S4b).9 For example, at the doping
level of 30 wt%, the phase diagram obtained upon heating displayed an
unknown phase below 54 °C and a crystalline phase at 54−65 °C.
(11) Chandrasekhar, S. In Handbook of Liquid Crystals: Low Molecular
Weight Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.
W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol.1, Chap. VIII, pp
749−780.
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